Hydrogenated Microcrystalline Silicon Single-Junction and Multi-Junction Solar Cells

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Hydrogenated Microcrystalline Silicon Single-Junction and Multi-Junction Solar Cells Baojie Yan, Guozhen Yue, Jeffrey Yang, Arindam Banerjee, and Subhendu Guha United Solar Systems Corp., Troy, MI 48084, U.S.A. ABSTRACT This paper summarizes our recent studies of hydrogenated microcrystalline silicon (µc-Si:H) solar cells as a potential substitute for hydrogenated silicon germanium alloy (a-SiGe:H) bottom cells in multi-junction structures. Conventional radio frequency (RF) glow discharge is used to deposit hydrogenated amorphous silicon (a-Si:H) and µc-Si:H at low rates (~ 1 Å/s), searching for the highest efficiency. We have achieved an initial active-area efficiency of 13.0% and stable efficiency of 11.2% using an a-Si:H/µc-Si:H double-junction structure. Modified very high frequency (MVHF) glow discharge is used to deposit a-Si:H and µc-Si:H at high rates (~ 3-10 Å/s) for comparison with our a-Si:H/a-SiGe:H/a-SiGe:H triple-junction production technology. The deposition time for the µc-Si:H intrinsic (i) layer in the bottom cell should be less than 30 minutes in order to be acceptable for mass production. To date, an initial active-area efficiency of 12.3% has been achieved with the bottom cell deposited in 50 minutes. By increasing the deposition rate and reducing the bottom cell thickness, we have achieved an initial active-area efficiency of 11.4% with the bottom cell i layer deposited in 30 minutes. The cell stabilized to 10.4% after prolonged light soaking. We will address issues related to µc-Si:H material, solar cell design, solar cell analysis, and stability. INTRODUCTION Using the µc-Si:H cell as a potential substitute for the narrow bandgap a-SiGe:H bottom cell in a multi-junction structure has attracted significant attention in the last decade [1-10]. First, the material is reportedly stable against light soaking [1, 2]. Second, deposition of µc-Si:H films does not require the expensive GeH4 source gas; less expensive SiH4 gas is used. Third, µc-Si:H material has an excellent long wavelength response that makes it eminently suitable for the bottom cell in a multi-junction structure. On the other hand, the µc-Si:H solar cell has some drawbacks. Due to the indirect bandgap, the maximum absorption coefficient of µc-Si:H is lower than that of a-Si:H or a-SiGe:H. Therefore, a thick i layer is necessary to obtain a high photocurrent. Thus, the deposition rate of µc-Si:H should be much higher than that of a-SiGe:H. Furthermore, µc-Si:H materials are usually made with high hydrogen dilution. It is known that the deposition rate decreases with increasing hydrogen dilution. Therefore, the challenge is to deposit good quality µc-Si:H i layers at high deposition rates. Many studies have been carried out on high rate deposition of µc-Si:H films. It is difficult to deposit good quality µc-Si:H films at a high deposition rate using conventional RF glow discharge. Meier et al. found that by using a VHF glow discharge technique, the deposition rates of a-Si:H and µc-Si:H can be increased significantly. They suc

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